1. Field of the Invention
This invention relates to welding stainless steel. More particularly, this invention relates to welding high chromium austenitic stainless steel pipe using a shielding gas.
2. Discussion of the Background
Bore welding techniques used to weld low sulfur, high chromium austenitic stainless steel pipes together, or to weld a pipe to a pipe plate, typically use 100 mole % of argon gas as a shielding gas. However, when the high chromium austenitic stainless steel pipe is 25 Cr-22 Ni-2 Mo stainless steel (ASME standardized material UNS S31050), 24 Cr-14 Ni-1 Mo stainless steel (JIS standardized material SUS 317J2) or the like, many problems arise. For example, in joining these steel pipes the welding often provides insufficient weld strength and corrosion resistance for practical use. This is because the content of sulfur, which is typically used to improve weld penetration, is only about 0.001% by weight in these steels, and pipes formed from these steels are often rather thick.
The present invention provides a method of welding stainless steel in which a shielding gas mixture including 88 to 98.9 mole % Ar, 1 to 10 mole % H2 and 0.1 to 2.0 mole % CO2 is used to shield and protect the stainless steel from contamination and oxidizing gases. The method is particularly suited to welding high chromium austenitic stainless steels. Even when welding thick pipes of stainless steels having low sulfur contents (e.g., 0.005% by weight or less), excellent weld penetration is achieved. Welds produced according to the present invention have enough strength, corrosion resistance and durability even under very sever conditions.